How do i extrude "screws" from my model
While it is possible to make a round peg fit a square hole in Blender, that's generally more difficult than it's worth. 

If you search "Cutting round holes Blender," there are some good resources out there for how to make a round element match up with a square element.

However, it's usually much more convenient to just model the greeble separately. Unless you have some specific reason that it has to be attached, I'd highly recommend this method. Keep in mind that you don't need to model a whole screw. You don't even have to model the whole head of the screw. You only need to model the part that will be seen, which is a tiny sliver of a disk with a slit in it.

If you're wanting to model any part of the hole or dent that the screw goes into on the weapon, it might be enough to just subdivide that area and "push" it in a little. If you want something more than that, a Boolean modifier would be a great way to cut out the part of the hole that you're interested in seeing.

I'll try to upload some images later today, if that helps.